Hydrographic Survey single beam echo sounder
Echo sounding is a type of sonar used to determine the depth of water by transmitting acoustic waves into water. The time interval between emission and return of a pulse is recorded, which is used to determine the depth of water along with the speed of sound in water at the time. This information is then typically used for navigation purposes or in order to obtain depths for charting purposes. Echo sounding can also refer to hydroacoustic “echo sounders” defined as active sound in water (sonar) used to study fish. Hydroacoustic assessments have traditionally employed mobile surveys from boats to evaluate fish biomass and spatial distributions. Conversely, fixed-location techniques use stationary transducers to monitor passing fish.
In addition to mapping out the subsurface geology, the single beam echosounder is often useful to generate detailed bathymetric models of the target area. Many of the places we core are not mapped in detail (ponds or lakes, or near shore regions removed from navigable waterways). Mapping lets us pinpoint depositional centers (basins and sub basins) but also gives us insight into the processes that have shaped the site and that act on the sediment.
The word sounding is used for all types of depth measurements, including those that don’t use sound, and is unrelated in origin to the word sound in the sense of noise or tones. Echo sounding is a more rapid method of measuring depth than the previous technique of lowering a sounding line until it touched bottom.
Distance is measured by multiplying half the time from the signal’s outgoing pulse to its return by the speed of sound in the water, which is approximately 1.5 kilometres per second [T÷2×(4700 feet per second or 1.5 kil per second )] For precise applications of echo sounding, such as hydrography, the speed of sound must also be measured typically by deploying a sound velocity probe into the water. Echo sounding is effectively a special purpose application of sonar used to locate the bottom. Since a traditional pre-SI unit of water depth was the fathom, an instrument used for determining water depth is sometimes called a fathometer. The first practical fathometer was invented by Herbert Grove Dorsey and patented in 1928. Single beam echosounders
Most charted ocean depths use an average or standard sound speed. Where greater accuracy is required average and even seasonal standards may be applied to ocean regions. For high accuracy depths, usually restricted to special purpose or scientific surveys, a sensor may be lowered to measure the temperature, pressure and salinity. These factors are used to calculate the actual sound speed in the local water column. This latter technique is regularly used by US Office of Coast Survey for navigational surveys of US coastal waters. See NOAA Field Procedures Manual, Office of Coast Survey website (http://www.nauticalcharts.noaa.gov/hsd/fpm/fpm.htm).
The required precision and accuracy of the hydrographic echo sounder is defined by the requirements of the International Hydrographic Organization (IHO) for surveys that are to be undertaken to IHO standards. These values are contained within IHO publication S44.
In order to meet these standards, the surveyor must consider not only the vertical and horizontal accuracy of the echo sounder and transducer, but the survey system as a whole. A motion sensor may be used, specifically the heave component (in single beam echosounding) to reduce soundings for the motion of the vessel experienced on the water’s surface. Once all of the uncertainties of each sensor are established, the hydrographer will create an uncertainty budget to determine whether the survey system meets the requirements laid down by IHO.
Different hydrographic organizations will have their own set of field procedures and manuals to guide their surveyors to meet the required standards. Two examples are the US Army Corps of Engineers publication EM110-2-1003, and the NOAA ‘Field Procedures Manual’
Single beam echo sounders are used To accurately position and interpret the data received by the MBES, the INS uses two Global Navigation Satellite System antennae and an inertial motion unit to provide position in three-dimensional space and measure the heave, pitch, roll, and heading of the vessel (and, thereby, the MBES). A connection to a source of real-time kinematic corrections often is established to improve real-time display of a survey. Whether or not a source of real-time kinematic corrections is used during a survey, data from the INS typically are postprocessed to mitigate the effects of degraded positional accuracy of the vessel during the survey. After the survey is completed, the acquired data from the MBES are processed to remove data spikes and other spurious points in the MBES soundings, georeferenced using the postprocessed INS data, and visualized as a triangulated irregular network surface or a point cloud (figs. 1, 2). The various components of the MBES mapping system are described in detail in studies of the Missouri and Mississippi Rivers in Missouri (Huizinga, 2016, 2017; Huizinga and others, 2010). Applications of the Multibeam Echosounder Mapping System Channel-Bed Scour Scour in river channels is the removal of channel-bed and bank material by flowing water and is the leading cause of bridge failures in the United States (Richardson and Davis, 2001). Scour at a bridge site is the result of short- and long-term geomorphic processes and local effects caused by elements of the structure (pier, footing) in or adjacent to the waterway (Richardson and Davis, 2001; Huizinga and Rydlund, 2004). Scour processes can be exacerbated during high-flow conditions because velocity and depth typically increase. Because the effects of scour can be severe and dangerous, bridges and other structures over waterways are inspected routinely. Multibeam surveys around bridges can reveal the short- and long-term effects of bed scour near the bridge structures (figs. 4, 5).
Single beam echo sounders (SBES), also known as depth sounders or fathometers determine water depth by measuring the travel time of a short sonar pulse, or “ping”. The sonar ping is emitted from a transducer positioned just below the water surface, and the SBES listens for the return echo from the bottom. In reality, the sonar energy will be reflected by anything that may be in the path of the sound – fish, debris, aquatic vegetation and suspended sediment. Hydrographic survey grade single beam echo sounders are able to provide accurate bottom depths by distinguishing the real bottom from any spurious signals in the returned echo. True survey-grade hydrographic single beam echosounders record a digital water column echogram or echo envelope, that provides a graphical representation of the return echo. Historically this information was presented on a paper chart recorder using thermal paper to provide the surveyor with a means to qualify sounding accuracy. SBES may use various different sonar frequencies; typically 200 kHz is used in shallow water under 100m. As the attenuation of sound in water decreases at lower frequencies, 24-33 kHz is commonly used for deeper water surveys. Often, two frequencies are combined for convenience into a single dual frequency transducer, eg 33/200 kHz. For surveys when suspended particulates are very high, usually when dredging is taking place, the low frequency sonar is able to penetrate the thick resuspended layer and measure the undisturbed hard bottom beneath. Transducers may be selected with different beam widths, which determines the size of the ping footprint on the bottom. Single beam echosounder Narrower beam transducers provide a smaller personified area and therefore present a depth measurement at a more discrete point under the survey vessel. To determine the exact position of bottom features, narrower beam width transducers are desirable. Inexpensive depth sounders may offer a very wide beam width, presenting a low potential for accurate depth measurement. Lower frequency transducers typically have a wider beam width than high frequency options; the transducer needs to be larger to generate a directional beam as the frequency decreases. Single beam echo sounders offer significant cost savings compared to multibeam echosounder systems and are especially useful in very shallow water, under 5-10m depth. Results from single beam echosounders are easier to interpret, far less time-consuming to edit, and the SBES equipment may be operated by less experienced personnel.
Single beam echo sounder sonars use a single transducer to transmit and receive acoustic energy signals, or “pings”. There are two types of single-beam sonar, profiling and imaging. Imaging sonar is primarily used for visual interpretation and uses a fan-shaped acoustic beam to scan a specified area or target. Profiling sonar is primarily used for quantitative measurements where a narrow, conically shaped beam generates a single point for each ping. With single-beam scanning sonar, the smaller and narrower the horizontal beam, the better the resolution.
PROFILING AND IMAGING
Single-beam sonars use a single transducer to transmit and receive acoustic energy signals, or “pings”. There are two types of single-beam sonar, profiling and imaging. Imaging sonar is primarily used for visual interpretation and uses a fan-shaped acoustic beam to scan a specified area or target. Profiling sonar is primarily used for quantitative measurements where a narrow, conically shaped beam generates a single point for each ping. With single-beam scanning sonar, the smaller and narrower the horizontal beam, the better the resolution.
Unmanned survey vessels or unmanned survey boats like the BathyCat can be RC remote controlled or autonomous using an autopilot system like the bathynav. Using unmanned survey boats keeps personnel safely on shore while conducting a single beam survey. Unmanned survey boats combined with GPS and single beam echosounders are a safe effective way to map waterways. Also unmanned RC survey boats like the bathycat are easy to deploy and transport.